http://uscis.gov/

How Do I Get a Work Permit (Employment Authorization Document)?
http://uscis.gov/graphics/howdoi/ead.htm

Step 1. Filing I-765
I-765, Application for Employment Authorization
http://uscis.gov/graphics/formsfee/forms/i-765.htm

You may be eligible to file Form I-765 electronically.
E-Filing
http://uscis.gov/graphics/formsfee/forms/efiling.htm

Filing Fee $175

Step 2. Make an appointment for USCIS Application Support Center
Photographs and Fingerprints for Form I-765,
Application for Employment Authorization
http://uscis.gov/graphics/formsfee/forms/e-photo.htm#I-765

Scheduling An Appointment for Fingerprinting and Photographs.
http://uscis.gov/graphics/formsfee/forms/e-photo_schedule.htm

Note: Waiting time to speak with representative takes quite long time, so I recommend make it early in the morning as soon as you filed I-765.
(Please make a phone call at any time and find out when is their business hours first.)

Step 3. Visit USCIS Application Support Center with documents required
http://uscis.gov/graphics/fieldoffices/aboutus.htm#serviceCenter
http://uscis.gov/graphics/fieldoffices/ascs/asc.htm#california

Renewal EAD:
You should apply for a renewal EAD six months before your original EAD expires.

Q. Do I have to pay for filing fee every time I renew my work permit?
A. Yes.

FAQ
http://uscis.gov/graphics/faqsgen.htm#auth

Hope this helps,